1. Field of the Invention
This invention relates to a mechanical mounting for a zoom lens, and more particularly to a structure for restraining a rotatable cam sleeve having variator and compensator control camming slots from axial movement when it is fitted in a tubular body.
2. Description of the Prior Art
The zoom lenses for TV cameras and motion picture cameras have generally been mounted in a mechanism such that a rotatable cam sleeve for changing the axial positions of the variator and compensator is fitted onto the inner surface of a body tube, and, in order to restrain the aforesaid cam sleeve from axial movement, the aforesaid body tube or a member fixedly secured thereto, is provided with a pair of positioning collars at the front and rear ends of the entire length of the cam sleeve. For example, U.S. Pat. No. 3,610,733 discloses such a zoom lens mounting.
The use of this prior art restraining means, however, requires that the clearance between the cam sleeve and the positioning collars be made optimum. If the clearance is too large, the imaging performance is lowered by the change in the axial position of the cam sleeve. If it is too small, the driving torque is caused to increase, causing inconvenience to the photographer. Particularly when the body tube and the cam sleeve are made of different materials, it cannot be avoided for the clearance necessarily to be at a larger value due to the discrepancy in coefficient of linear expansion by temperature change. To absorb the backlash of the cam sleeve owing to the clearance, use must be made of an axially tensioned spring member between the tubular body and the cam sleeve. This produces a disadvantage of increasing the driving torque for the zooming operation.
It is, therefore, an object of the present invention to eliminate the above-described drawbacks of the prior art and to provide a mechanical mounting for a zoom lens with means making it possible to limit the axial backlash of the cam sleeve to a minimum and also to maintain the variation of the driving torque with variation in the ambient temperature at a minimum so that zooming can be performed always by a small driving torque.